Walla Walla County, WA Accessory Structures: ADU Impact Fees (2026)

Section 3.08.110 of the Walla Walla County Code lists "Accessory Dwelling Unit" as its own $744.00 line on the Community Development Department's planning application fee schedule, a flat charge collected separately from the building permit valuation fee that applies once construction actually starts. The county has no GMA-style impact fee tied to school, park, or road capacity specifically for ADUs; the $744 planning fee, plus the standard building permit fee, is the entire county charge.

Section 17.08.015 defines an accessory dwelling unit as a unit within the primary residence or an accessory building for family members or guests, and treats "guesthouse" as the same thing. The primary residence must already be at least 800 square feet before an ADU can be added, and the ADU itself cannot exceed 1,100 square feet or 50 percent of the primary home's floor area, whichever is smaller. Only one ADU is allowed per lot, mobile homes and RVs cannot serve as the unit, and it must share at least four of six listed features with the main house, including road access, septic, water, utility meters, yard, or parking. Either the primary house or the ADU must be owner-occupied at least six months a year, and the owner cannot collect rent on the unit they occupy, though the ADU itself may be rented out. One extra off-street parking space is required beyond what the main house already needs. Starting construction before the county issues a permit doubles the fee under Section 15.04.031.